netdev - Re: Three questions about busy poll

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Date:   Fri, 15 Feb 2019 17:46:23 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        sridhar.samudrala@...el.com,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Three questions about busy poll

> > > 2. Why there is no socket option for sysctl.net.busy_poll? Clearly
> > > sysctl_net_busy_poll is global and SO_BUSY_POLL only works for
> > > sysctl.net.busy_read.
> >
> > I guess because of how sock_poll works. In that case it is not needed.
> > The poll duration applies more to the pollset than any of the
> > individual sockets, too.
>
>
> Good point, it's probably like struct eventpoll vs. struct epitem.
>
> The reason why I am looking for a per-socket tuning is to minimize
> the impact of setting busy_poll. I don't know if it is possible to somehow
> make this per-socket via epoll interfaces, perhaps fundamentally
> it is impossible?

I think it may be possible. The way busy_read and busy_poll work
in sock_poll is that the sum of all (per socket tunable) busy_read
durations on the sockets in the pollset is ~bound by (global) busy_poll.

The epoll implementation is restricted in the sense that it polls only
on one napi_id at a time. Alongside setting ep->napi_id in
ep_set_busy_poll_napi_id, we could also set a new ep field takes
the min of the global busy_poll and sk->sk_ll_usec.

Though I guess you want to be able to poll on a given pollset
without setting the global sysctl_net_busy_poll at all? That
would be a useful feature both for epoll and poll/select. But
definitely requires refining net_busy_loop_on() to optionally
take some state derived from the sockets in the (e)pollset.